Apparatus for electrolyzing sulfuric acid and method for electrolyzing sulfuric acid

ABSTRACT

The present invention relates to an apparatus to produce oxidizing agent-rich sulfuric acid by electrolysis of sulfuric acid. More specifically, it relates to the apparatus by which dilute sulfuric acid of the specified temperature and concentration is formed within the electrolysis system and then, by electrolysis of the formed dilute sulfuric acid, electrolytic sulfuric acid containing richly oxidizing agent is formed at a high efficiency and safely under the temperature control. 
     The apparatus for electrolyzing sulfuric acid and the method for electrolyzing sulfuric acid by the present invention comprise the anode side dilute sulfuric acid generation loop A in which concentrated sulfuric acid, as feed material, is diluted and controlled to the specified temperature and concentration, and the anode side electrolytic sulfuric acid generation loop B in which electrolytic sulfuric acid is formed by electrolysis of dilute sulfuric acid with the temperature and concentration controlled to the specified range in the anode side dilute sulfuric acid generation loop A, in, at least, the anode side electrolysis part of the apparatus for electrolyzing sulfuric acid having the anode side electrolysis part and the cathode side electrolysis part.

TECHNICAL FIELD

The present invention relates to an apparatus for electrolyzing sulfuric acid and a method for electrolyzing sulfuric acid for producing oxidizing agent-rich electrolytic sulfuric acid through electrolysis of sulfuric acid.

More in detail, the present invention relates to the apparatus for electrolyzing sulfuric acid and the method for electrolyzing sulfuric acid for producing electrolytic sulfuric acid containing oxidizing agent at a high efficiency and safely in such a manner that dilute sulfuric acid for which temperature and concentration are controlled is formed in the apparatus for electrolyzing sulfuric acid and the temperature- and concentration-controlled dilute sulfuric acid is electrolyzed.

BACKGROUND ART

Conventionally, persulfuric acid has been used in various manufacturing and inspection processes as chemical agent, such as pre-treatment agent for electrolytic metal plating, etching agent, oxidizing agent in chemical and mechanical polishing treatment for semiconductor device manufacturing, oxidizing agent for organic substance in wet analyses, and cleaning agent for silicon wafer. Persulfuric acid, which is called “oxidizing agent”, is known to be formed by electrolysis of sulfuric acid, and has been electrolytically manufactured in an industrial scale.

In the present invention, the “oxidizing agent” indicates persulfuric acid which names generically peroxodisulfuric acid and peroxomonosulfuric acid and hydrogen peroxide and the “electrolytic sulfuric acid” indicates the substance which contains oxidizing agent produced through the electrolysis of sulfuric acid and unreacted sulfuric acid.

The oxidizing agent and the electrolytic sulfuric acid containing unreacting sulfuric acid (hereinafter, simply called “electrolytic sulfuric acid”) formed by a system to electrolyze sulfuric acid are applied to remove resist, pollutant organic matter or metals in the semiconductor manufacturing process. For these applications, it is known that the higher the concentration of oxidizing agent is, the higher the effect of removal becomes. Then, it is required for the apparatus for electrolyzing sulfuric acid to have such features that electrolytic sulfuric acid containing oxidizing agent is formed at a higher concentration, the current efficiency to form oxidizing agent through electrolysis is higher and the degradability of formed oxidizing agent is low. In the sulfuric acid electrolysis, in order to form electrolytic sulfuric acid containing oxidizing agent at a high concentration, enhance the current efficiency of oxidizing agent and reduce degradability of oxidizing agent, supply of sulfuric acid at a low concentration controlled to an intended density is required for the apparatus for electrolyzing sulfuric acid.

Generally speaking, however, sulfuric acid commercially available is concentrated sulfuric acid at the density of 98% or 96%, and therefore when sulfuric acid diluted to that of a low density, which is called dilute sulfuric acid is to be supplied to the apparatus for electrolyzing sulfuric acid, the chemicals supply equipment of the plant must have an exclusive storage tank as additional unit and supply piping, requiring a large amount of equipment cost. In addition, sulfuric acid of a low concentration is larger in volume compared with concentrated sulfuric acid, and the transportation cost of the chemical will be increased compared with that of concentrated sulfuric acid.

If the concentration of sulfuric acid can be controlled at a high efficiency within the apparatus for electrolyzing sulfuric acid, sulfuric acid electrolysis to obtain oxidizing agent at a high efficiency through electrolysis of sulfuric acid of a low concentration is realized, with the costs for equipment, transportation, etc. for preparing dilute sulfuric acid being reduced to a minimum. Moreover, if the equipments and lines for generating dilute sulfuric acid from concentrated sulfuric acid are made in common use with those for generating electrolytic sulfuric acid containing oxidizing agent from dilute sulfuric acid as much as possible, downsizing and simplification of the apparatus for electrolyzing sulfuric acid are achieved.

Paragraph 0011 of PTL 1 which describes generation of persulfuric acid through electrolysis of sulfuric acid in an electrolytic cell discloses, “the current efficiency of persulfuric acid can be enhanced by specifying the concentration range of sulfuric acid to be used for generation of persulfuric acid at a low sulfuric acid concentration of 2˜11 mol/L.”

Paragraph 0026 of PTL 2 proposing a persulfuric acid supply system discloses, “regarding the range of sulfuric acid concentration in the electrolyte supplied to the electrolysis system, the current efficiency of persulfuric acid can be enhanced by controlling the concentration of sulfuric acid to as low as 10˜18M (mol/L).”

Paragraph 0012 and Paragraph 0018 of PTL 3 disclose, “a method to enhance the current efficiency to form electrolytic sulfuric acid and form oxidizing agent efficiently and stably by applying sulfuric acid of a different concentration as electrolyte.”

However, there is no disclosure in the methods described in PTLs 1˜3 about the controlling method for concentration of sulfuric acid, although it is disclosed that electrolyzing low concentration sulfuric acid gives a high current efficiency.

In general, it is necessary for the production of dilute sulfuric acid at a low concentration that concentrated sulfuric acid is mixed with pure water to adjust the concentration of sulfuric acid as required. While sulfuric acid is mixed with pure water, a large quantity of heat of dilution occurs, together with a large quantity of vapor or mist derived from boiling or heat of dilution. For this reason, if exhaust from the tank or equipment controlling the concentration of sulfuric acid is introduced to exhaust facilities or a scrubber without any treatment, sulfuric acid enters the exhaust facilities or the scrubber, which causes problems of corrosion or performance degradation.

PTL 4 discloses an application of gas liquid separation means as a removing method of sulfuric acid containing electrolytically generated gas generated from the electrolysis system.

However, PTL 4 discloses neither the removal of vapor or mist occurring at controlling the sulfuric acid concentration nor the method for controlling the sulfuric acid concentration in spite that the quantity of the sulfuric acid in the vapor or mist generated at controlling the sulfuric acid concentration in the system is larger than that of the sulfuric acid contained in the electrolytically generated gas.

PTL 5 discloses the method of generating persulfuric acid in which the sulfuric acid used for cleaning is re-concentrated, and re-electrolyzed after dilution and cool down. In this method, however, sulfuric acid used for cleaning supplied at a low concentration is re-concentrated and therefore, the cleanliness is not uniform and the safety is problematic.

CITATION LIST Patent Literature

-   [PTL 1] JP2008-66464(A) -   [PTL 2] JP2008-111184(A) -   [PTL 3] JP2010-34521(A) -   [PTL 4] JP2007-262532(A) -   [PTL 5] JP2008-244310(A)

SUMMARY OF INVENTION Technical Problem

The present invention aims to provide an apparatus for electrolyzing sulfuric acid and a method for electrolyzing sulfuric acid by which oxidizing agent is electrolytically produced at a high current efficiency, safely and stably for a long time of operation in such a manner that the heat of dilution occurring at the time of diluting concentrated sulfuric acid to sulfuric acid of a low concentration and the heat occurring at the time of electrolysis are removed, electrolysis conditions for producing oxidizing agent at a high current efficiency are prepared, generation of mist or vapor caused by the heat of dilution is suppressed, and condensate droplet of sulfuric acid derived from the mist or vapor got mixed with the exhaust line is removed.

Solution to Problem

In order to solve the afore-mentioned problems, the present invention provides an apparatus for electrolyzing sulfuric acid characterized in that in the apparatus for electrolyzing sulfuric acid 1 comprising the anode side electrolysis part 20 and the cathode side electrolysis part 23,

the anode side dilute sulfuric acid generation loop A in which concentrated sulfuric acid, as feed material, is diluted and controlled to a specified temperature and concentration, and the anode side electrolytic sulfuric acid generation loop B in which the dilute sulfuric acid generated in the anode side dilute sulfuric acid generation loop A is electrolyzed to form electrolytic sulfuric acid and the formed electrolytic sulfuric acid is controlled to a specified temperature and concentration are provided at least in the anode side electrolysis part 20;

in the anode side dilute sulfuric acid generation loop A, the anode side tank 31, the anode side concentrated sulfuric acid supply part 32 and the anode side cooler 34 are disposed in this order constituting a loop being connected by the anode side by-pass pipe 36, to which the anode side pure water supply pipe 10 is connected to feed pure water to the anode side dilute sulfuric acid generation loop A at any point wherever in the anode side dilute sulfuric acid generation loop A;

the anode side concentrated sulfuric acid supply pipe 27 is connected to feed concentrated sulfuric acid to the anode side concentrated sulfuric acid supply part 32;

in the anode side electrolytic sulfuric acid generation loop B, the anode side tank 31 and the anode compartment 4 provided internally with the anode 3 of the electrolytic cell 2 comprising the anode compartment 4 and the cathode compartment 7 separated by the diaphragm 5 constitute a loop being connected by the anode side circulation pipe 37;

the concentrated sulfuric acid supplied to the anode side concentrated sulfuric acid supply part 32 from the anode side concentrated sulfuric acid supply pipe 27 is diluted with pure water supplied by the anode side pure water supply pipe 10;

the diluted sulfuric acid of a low concentration is controlled to a specified temperature and concentration while being circulated in the anode side dilute sulfuric acid generation loop A to form dilute sulfuric acid of the specified temperature and concentration;

the formed dilute sulfuric acid is supplied to the anode compartment 4 of the electrolytic cell 2 via the anode side circulation pipe 37 constituting the anode side electrolytic sulfuric acid generation loop B to form electrolytic sulfuric acid in the anode compartment 4;

the formed electrolytic sulfuric acid is controlled to a specified temperature and concentration while being circulated in the anode side electrolytic sulfuric acid generation loop B; and

-   -   electrolytic sulfuric acid of the specified temperature and         concentration is generated.

As the second solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid, characterized in that:

in the cathode side electrolysis part 23, the cathode side dilute sulfuric acid generation loop A′ in which concentrated sulfuric acid, as feed material, is diluted to sulfuric acid of a low concentration and the sulfuric acid of a low concentration is controlled to a specified temperature and concentration, and the cathode side electrolysis loop B′ in which the dilute sulfuric acid formed in the cathode side dilute sulfuric acid generation loop A′ is circulated through the cathode compartment 7 are provided;

in the cathode side dilute sulfuric acid generation loop A′, the cathode side tank 38, the cathode side concentrated sulfuric acid supply part 39 and the cathode side cooler 41 are disposed in this order, constituting a loop being connected by the cathode side by-pass pipe 43, to which the cathode side pure water supply pipe 12 is connected to feed pure water to the cathode side dilute sulfuric acid generation loop A′ at any point wherever in the cathode side dilute sulfuric acid generation loop A′ and the cathode side concentrated sulfuric acid supply pipe 29 is connected to feed concentrated sulfuric acid to the cathode side concentrated sulfuric acid supply part 39;

in the cathode side electrolysis loop B′, the cathode side tank 38 and the cathode compartment 7 provided internally with the cathode 6 of the electrolytic cell 2 comprising the anode compartment 4 and the cathode compartment 7 separated by the diaphragm 5 constitute a loop being connected by the cathode side circulation pipe 44;

the concentrated sulfuric acid supplied to the cathode side concentrated sulfuric acid supply part 39 from the cathode side concentrated sulfuric acid supply pipe 29 is diluted with pure water supplied via the cathode side pure water supply pipe 12;

the diluted sulfuric acid of a low concentration is controlled to a specified temperature and concentration while being circulated in the cathode side dilute sulfuric acid generation loop A′ to form dilute sulfuric acid of the specified temperature and concentration;

the formed dilute sulfuric acid is supplied to the cathode compartment 4 of the electrolytic cell 2 via the cathode side circulation pipe 44 constituting the cathode side electrolysis loop B′; and

the dilute sulfuric acid controlled to the specified temperature and concentration is electrolyzed while being circulated in the cathode side electrolysis loop B′

As the third solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid, featuring that above the anode side tank 31, the anode side gas-liquid separator 91 and the anode side mist separator 92 are connected sequentially in series via the anode side gas vent pipe 102, and the respective bottoms of the anode side gas-liquid separator 91 and the anode side mist separator 92 are provided with a draining means of a communicating vessel structure for the common use by the anode side gas-liquid separator 91 and the anode side mist separator 92 to drain liquid accumulated at the bottom of the respective separators.

As the fourth solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid, featuring that above the anode side tank 31, the anode side gas-liquid separator 91 and the anode side mist separator 92 are connected sequentially in series via the anode side gas vent pipe 102, and the respective bottoms of the anode side gas-liquid separator 91 and the anode side mist separator 92 are provided with a draining means of a communicating vessel structure for the common use by the anode side gas-liquid separator 91 and the anode side mist separator 92 to drain liquid accumulated at the bottom of the respective separators,

and above the cathode side tank 38, the cathode side gas-liquid separator 96 and the cathode side mist separator 97 are connected sequentially in series via the cathode side gas vent pipe 103, and the respective bottoms of the cathode side gas-liquid separator 96 and the cathode side mist separator 97 are provided with a draining means of a communicating vessel structure for the common use by the cathode side gas-liquid separator 96 and the cathode side mist separator 97 to drain liquid accumulated at the bottom of the respective separators.

As the fifth solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid featuring that the ozone decomposition mechanism 93 is connected to the anode side mist separator 92.

As the sixth solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid featuring that the hydrogen treatment mechanism is provided to the cathode side mist separator 97.

As the seventh solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid featuring to have such configuration that more than one of anode side tank are installed in parallel in the anode side dilute sulfuric acid generation loop A, generated electrolytic sulfuric acid containing oxidizing agent is once stored in one of the tanks, and then, the valve is switched to other anode side tanks where electrolytic sulfuric acid containing oxidizing agent of the specified concentration is formed.

As the eighth solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid featuring to have such configuration that while electrolytic sulfuric acid containing oxidizing agent of the specified concentration stored in one anode side tank is being transferred to a location of use outside the apparatus for electrolyzing sulfuric acid, electrolytic sulfuric acid containing oxidizing agent of the specified concentration is formed in another anode side tank.

As the ninth solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid featuring that the anode 3 is a conductive diamond electrode.

As the tenth solution to solve the afore-mentioned problems, the present invention provides the apparatus for electrolyzing sulfuric acid featuring that the diaphragm 5 is a fluororesin type cation exchange membrane or a hydrophilically treated porous fluororesin membrane.

As the eleventh solution to solve the afore-mentioned problems, the present invention provides a method for electrolyzing sulfuric acid characterized in that electrolytic sulfuric acid controlled to a specified temperature and concentration is formed by applying any one of the afore-mentioned apparatuses for electrolyzing sulfuric acid

As the twelfth solution to solve the afore-mentioned problems, the present invention provides a method for electrolyzing sulfuric acid characterized in that any one of the afore-mentioned apparatuses for electrolyzing sulfuric acid is applied, a porous fluororesin membrane is applied as the diaphragm 5, and overflowing of the cathode side tank 38 is prevented by draining periodically or draining by the specified quantity when the liquid level of the cathode side tank 38 has reached the specified point because of an increase of the dilute sulfuric acid solution circulating in the cathode side electrolysis loop B′ of the cathode side electrolysis part 23 due to entrained water brought by cation at the time of passing through the porous fluororesin membrane.

As the thirteenth solution to solve the afore-mentioned problems, the present invention provides the method for electrolyzing sulfuric acid characterized in that any one of the afore-mentioned apparatuses for electrolyzing sulfuric acid is applied, a porous fluororesin membrane is applied as the diaphragm 5, and concentrated sulfuric acid is replenished to the cathode side concentrated sulfuric acid supply part 39 in order to maintain a certain range of dilute sulfuric acid concentration when the sulfuric acid concentration of the cathode side dilute sulfuric acid solution formed in the loop A′ of the cathode side electrolysis part 23 has decreased to or below the specified level of concentration due to entrained water brought by cation at the time of passing through the porous fluororesin membrane.

As the fourteenth solution to solve the afore-mentioned problems, the present invention provides any one of the afore-mentioned methods for electrolyzing sulfuric acid featuring that the temperature of dilute sulfuric acid before electrolysis in the anode side dilute sulfuric acid generation loop A of the anode side electrolysis part 20 or in the cathode side dilute sulfuric acid generation loop A′ of the cathode side electrolysis part 23 is controlled to 30 degrees Celsius or less.

As the fifteenth solution to solve the afore-mentioned problems, the present invention provides any one of the afore-mentioned methods for electrolyzing sulfuric acid featuring that the temperature of electrolyzed electrolyte in the anode side electrolytic sulfuric acid generation loop B of the anode side electrolysis part 20 or in the cathode side electrolysis loop B′ of the cathode side electrolysis part 23 is controlled to 30 degrees Celsius or less.

As the sixteenth solution to solve the afore-mentioned problems, the present invention provides any one of the afore-mentioned methods for electrolyzing sulfuric acid featuring that the sulfuric acid concentration of dilute sulfuric acid before electrolysis in the anode side dilute sulfuric acid generation loop A of the anode side electrolysis part 20 or in the cathode side dilute sulfuric acid generation loop A′ of the cathode side electrolysis part 23 is controlled to the range of 2˜10 mol/L.

ADVANTAGEOUS EFFECTS OF INVENTION

The apparatus for electrolyzing sulfuric acid and the method for electrolyzing sulfuric acid by the present invention can form dilute sulfuric acid controlled to an intended specification of temperature and concentration in the apparatus for electrolyzing sulfuric acid; by performing electrolysis of dilute sulfuric acid under controlled temperature, oxidizing agent-rich electrolytic sulfuric acid can be produced at a high current efficiency and safely; and electrolyte containing oxidizing agent at a high concentration can be produced at a high current efficiency, which has not been able to be accomplished by the prior art.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 An overall view of an example of the apparatus for electrolyzing sulfuric acid by the present invention

FIG. 2 A process drawing explaining each process of temperature control, concentration control, electrolysis, supply, drain treatment, etc. of sulfuric acid carried out by the apparatus for electrolyzing sulfuric acid in FIG. 1

FIG. 3 A block diagram showing the anode side electrolysis part 20 of another example of the apparatus for electrolyzing sulfuric acid by the present invention

FIG. 4 A process drawing explaining each process of temperature control, concentration control, electrolysis, supply, drain treatment, etc. of sulfuric acid carried out by the apparatus for electrolyzing sulfuric acid in FIG. 3

DESCRIPTION OF EMBODIMENTS

The following explains, in detail, an exemplified embodiment of the present invention in reference to the drawings.

FIG. 1 shows an example of the apparatus for electrolyzing sulfuric acid 1 by the present invention. The apparatus for electrolyzing sulfuric acid 1 comprises the anode side electrolysis part 20 and the cathode side electrolysis part 23, incorporating the electrolytic cell 2. The electrolytic cell 2 is divided into the anode compartment 4 and the cathode compartment 7 by the diaphragm 5. The anode 3 is installed in the anode compartment 4, and the cathode 6 is installed in the cathode compartment 7. The anode compartment 4 is provided in the anode side electrolysis part 20 of the apparatus for electrolyzing sulfuric acid 1. The present invention is characterized in that the anode side electrolysis part 20 is configured in the following way.

The anode side electrolysis part 20 comprises the anode side dilute sulfuric acid generation loop A and the anode side electrolytic sulfuric acid generation loop B. In the apparatus exemplified in FIG. 1, the anode side tank 31, the anode side concentrated sulfuric acid supply part 32, the anode side circulation pump 33, and the anode side cooler 34 are disposed in this order in the anode side dilute sulfuric acid generation loop A, being connected by the anode side by-pass pipe 36, constituting a loop. It is configured in such a way that the circulation of the anode side dilute sulfuric acid generation loop A can be suspended by the anode side by-pass valve 35 provided between the anode side cooler 34 and the anode side tank 31.

Besides, in the apparatus exemplified in FIG. 1, the anode side pure water supply pipe 10 is connected to the anode side tank 31, and the anode side concentrated sulfuric acid supply pipe 27 is connected to the anode side concentrated sulfuric acid supply part 32. Concentrated sulfuric acid supplied from the anode side concentrated sulfuric acid supply pipe 27 to the anode side concentrated sulfuric acid supply part 32 via the anode side concentrated sulfuric acid supply valve 28 is diluted with pure water supplied from the anode side pure water supply pipe 10 via the anode side pure water supply valve 11 to sulfuric acid of a low concentration in the anode side tank 31. Diluted sulfuric acid is controlled to the specified temperature and concentration while being circulated in the anode side dilute sulfuric acid generation loop A. The dilute sulfuric acid controlled to the specified temperature and concentration in the anode side dilute sulfuric acid generation loop A is supplied to the anode compartment 4 of the electrolytic cell 2 forming the anode side electrolytic sulfuric acid generation loop B, and electrolyzed. The anode side electrolytic sulfuric acid generation loop B is to be described in the latter part.

In the anode side dilute sulfuric acid generation loop A, pure water is supplied to the anode side tank 31, after quantitation by an integrating flow meter or the level gauge provided to the tank, which are not shown in FIG. 1. Applicable types of the integrating flow meter include those of ultrasonic wave, electromagnetic and coriolis. Supply or cease of supply of pure water is controlled by a controller based on the measurements or signals from the integrating flow meter or the liquid surface sensor. The anode side pure water supply pipe 10 can be connected at any part within the anode side dilute sulfuric acid generation loop A, not limited to the exemplified case of FIG. 1. Dilute sulfuric acid is circulated respectively in the anode side dilute sulfuric acid generation loop A or the anode side electrolytic sulfuric acid generation loop B by the open-close operation of the anode compartment inlet valve 21, the anode compartment outlet valve 22 and the anode side by-pass valve 35.

Similarly, the cathode compartment inlet valve 24 and the cathode compartment outlet valve 25 function.

In the anode side electrolytic sulfuric acid generation loop B, the anode compartment 4 of the electrolytic cell 2 is connected with the anode side tank 31 by the anode side circulation pipe 37, constituting a loop. Dilute sulfuric acid formed in the anode side dilute sulfuric acid generation loop A is circulated in the anode side electrolytic sulfuric acid generation loop B by the valves provided midway on the respective piping.

Dilute sulfuric acid subjected to the temperature and concentration control in the anode side dilute sulfuric acid generation loop A is electrolyzed to form electrolytic sulfuric acid in the anode side electrolytic sulfuric acid generation loop B, and the formed electrolytic sulfuric acid and the dilute sulfuric acid prepared in the anode side dilute sulfuric acid generation loop A are mixed while being circulated in the anode side electrolytic sulfuric acid generation loop B so that the electrolytic sulfuric acid is controlled to the specified temperature and concentration. For the piping or the wet part of equipment, materials which have corrosion-resistance to sulfuric acid or sulfuric acid containing oxidizing agent are required. Those include fluororesin such as polytetrafluoroethylene (PTFE) and tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA) and quartz.

The cathode side electrolysis part 23 of the apparatus for electrolyzing sulfuric acid 1 shown in FIG. 1 comprises, as with the anode side electrolysis part 20, the cathode side dilute sulfuric acid generation loop A′ which dilutes concentrated sulfuric acid to dilute sulfuric acid of the specified temperature and concentration and the cathode side electrolysis loop B′ which circulates the dilute sulfuric acid prepared in the cathode side dilute sulfuric acid generation loop A′ through the cathode compartment 7.

The cathode side dilute sulfuric acid generation loop A′ of the apparatus exemplified in FIG. 1 comprises the cathode side tank 38, the cathode side concentrated sulfuric acid supply part 39, the cathode side circulation pump 40, the cathode side cooler 41, the cathode side by-pass valve 42, the cathode side by-pass pipe 43, and valves provided midway on respective pipe lines. In the exemplified apparatus, the cathode side pure water supply pipe 12 is connected to the cathode side tank 38, and the cathode side concentrated sulfuric acid supply pipe 29 is connected to the cathode side concentrated sulfuric acid supply part 39. Concentrated sulfuric acid supplied from the cathode side concentrated sulfuric acid supply pipe 29 to the cathode side concentrated sulfuric acid supply part 39 via the cathode side concentrated sulfuric acid supply valve 30 is diluted with pure water supplied from the cathode side pure water supply pipe 12 to the cathode side tank 38 via the cathode side pure water supply valve 13 in the cathode side tank 38 to be dilute sulfuric acid of a low concentration. The dilute sulfuric acid is controlled to the specified temperature and concentration while being circulated in the cathode side dilute sulfuric acid generation loop A′, supplied to the cathode compartment 7 of the electrolytic cell 2 provided in the cathode side electrolysis loop B′ and electrolyzed.

It is recommended to configure that pure water is supplied to the cathode side tank 38, after quantitation by the integrating flow meter or the level gauge provided to each tank, which are not shown in the figures. Applicable types of the integrating flow meter include those of ultrasonic wave, electromagnetic and coriolis. Supply or cease of pure water is controlled by a controller based on the measurements or signals from the integrating flow meter or the liquid surface sensor. The cathode side pure water supply pipe 12 can be connected at any part within the cathode side dilute sulfuric acid generation loop A′.

For the piping or the wet part of equipment, materials which have corrosion-resistance to sulfuric acid or sulfuric acid containing oxidizing agent are required. Those include fluororesin such as PTFE and PFA, and quartz.

The cathode side electrolysis loop B′ comprises the cathode compartment 7 of the electrolytic cell 2, the cathode side circulation pipe 44, the cathode side tank 38 and valves provided midway on each pipe line, constituting a loop. In the cathode side electrolysis loop B′ of the cathode side electrolysis part 23, electrolysis of dilute sulfuric acid is performed, but the electrode reaction generates only hydrogen gas, without forming electrolytic sulfuric acid. Then, in the cathode side electrolysis loop B′, dilute sulfuric acid controlled to the intended temperature and concentration is circulated.

The anode side concentrated sulfuric acid supply part 32 and the cathode side concentrated sulfuric acid supply part 39 are able to be provided at any side of the inlet or outlet of the anode side circulation pump 33 and the cathode side circulation pump 40. However, in case that concentrated sulfuric acid is added to pressurized pure water, considerable heat and bubble will be generated at the point where pressure is built up due to dilution of concentrated sulfuric acid, leading to a possible further build up of pressure. In view of safety, the anode side concentrated sulfuric acid supply part 32 and the cathode side concentrated sulfuric acid supply part 39 are preferably provided at the inlet side of the anode side circulation pump 33 and the cathode side circulation pump 40. The concentration of sulfuric acid can be controlled by means of the volume ratio of the pure water supplied to the anode side dilute sulfuric acid generation loop A and the cathode side dilute sulfuric acid generation loop A′ to the concentrated sulfuric acid whose concentration is known. The volume of the respective liquids is controlled by an integrating flow meter, etc. through quantitation.

In the present invention, it is preferable to use conductive diamond electrode as the anode 3. Compared with the case in which Pt or PbO₂ is applied as electrode catalyst, conductive diamond electrode has advantages: a high current efficiency of persulfuric acid due to high oxygen overvoltage, chemically and mechanically high durability, and free from contamination by the anode, leading to formation of highly clean sulfuric acid solution, which is electrolyte and electrolytic sulfuric acid, which is product of electrolysis. From the reasons described, a conductive diamond electrode is preferably applied for the anode 3 of the electrolytic cell 2.

On the other hand, for the cathode 6, the conductive diamond electrode, which is superior in corrosion resistance, is preferable in view of cleanliness, but also applicable are such electrodes comprising platinum metal like platinum, valve metals such as titanium, zirconium, tantalum, and niobium, and carbon materials like graphite and glassy carbon, having corrosion resistance to sulfuric acid.

According to the inventors of the present invention, if the control of sulfuric acid concentration is carried out in the anode side electrolytic sulfuric acid generation loop B provided with the electrolytic cell 2 in the circulation system, the inside of the electrolytic cell 2 heats up due to heat of dilution to an degree which damages the diaphragm 5. To cope with this, the present invention applies such manner that liquid is not supplied to the electrolytic cell 2 while the concentration of sulfuric acid is being controlled, but is circulated and cooled in the anode side dilute sulfuric acid generation loop A, which allows the flow through the anode side by-pass pipe 36 in the anode side electrolysis part 20. For the same reason, also, in the cathode side electrolysis part 23, liquid is not supplied to the electrolytic cell 2, but is circulated and cooled in the cathode side dilute sulfuric acid generation loop A′, which allows the flow through the cathode side by-pass pipe 43, in order to control the concentration and temperature.

When the control of sulfuric acid concentration is made in the anode side dilute sulfuric acid generation loop A of the anode side electrolysis part 20 and the cathode side dilute sulfuric acid generation loop A′ of the cathode side electrolysis part 23, a large volume of vapor or mist may occur due to heat of dilution of sulfuric acid. In the present invention, it is preferable to install the anode side gas-liquid separator 91 and the anode side mist separator 92 downstream of the anode side gas vent pipe 102 connected to the anode side tank 31 in order to prevent the scrubber of electrolytically generated gas or units outside the apparatus from being corroded by vapor or mist entrained by the gas generated by electrolysis. Similarly, the cathode side gas-liquid separator 96 and the cathode side mist separator 97 are installed preferably downstream of the cathode side gas vent pipe 103 connected to the cathode side tank 38.

The explanation about the electrolytically generated gas generated by the electrolysis follows. Anode gas generated in the anode compartment 4 of the electrolytic cell 2 may contain toxic ozone. Therefore, it is desirable that the ozone decomposition catalyst is provided as the ozone decomposition mechanism 93 in the downstream of the anode side mist separator 92 so that ozone is reduced to harmless oxygen, or is vent outside the apparatus after being sufficiently diluted with air or inert gas. As the ozone decomposition catalyst, manganese dioxide is most commonly used, but manganese dioxide will dissolve when it comes in contact with acid solutions such as sulfuric acid of a low pH and ozone resolving power may be lost. In this case, even if the fluid which comes in contact is water, the ozone resolving power will be lost also when the surface of ozone decomposition catalyst is covered with water. Therefore, in order to treat ozone within the apparatus and run the apparatus safely, it is desirable to remove mist and vapor from electrolytically generated gas by the anode side gas-liquid separator 91 and the anode side mist separator 92. Except the case that vapor which may lead to mist or condensate is considered, in advance, to be supplied, the gas conducting piping is commonly made with metal pipes such as stainless steel. In this case, the pipes will be corroded in contact with sulfuric acid mist or condensate, and therefore, basically, it is necessary for the electrolytically generated gas containing such mist or condensate not to be drained outside the apparatus.

Hydrogen, which is the cathode gas occurring at the cathode compartment of the electrolytic cell 2 is inflammable and explosive. Therefore, it is necessary that the hydrogen gas is transformed into harmless water vapor by mixing with air for burning with the help of hydrogen combustion catalyst provided downstream of the cathode side mist separator 97 or to be sufficiently diluted with air or inert gas before discharging outside the apparatus. Hydrogen combustion catalyst has a function to eliminate hydrogen by burning hydrogen with air. The catalyst containing precious metal as an active ingredient for combustion is widely used. In general, if the surface of the catalyst is covered with liquid like water, hydrogen gas cannot get in contact with the catalyst and the combustion ability of hydrogen will be lost. Except the case that vapor which may lead to mist or condensate is considered to be supplied, the gas conducting piping is commonly made with metal pipes such as stainless steel. In this case, the pipes will be corroded in contact with sulfuric acid mist or condensate, and therefore, basically, it is necessary for the electrolytically generated gas containing such mist or condensate not to be drained outside the apparatus.

For the anode side gas-liquid separator 91 and the cathode side gas-liquid separator 96, the equipment is used with such mechanism that sulfuric acid is separated from the electrolytically generated gases by utilizing the difference of gravity between the electrolytically generated gas and the fluid in the electrolytically generated gas in the vessels like pipe or tank, or such mechanism that enough retention time is provided for the electrolytically generated gas in the vessel so that the mist falls down in the vessel. Applicable units for the anode side mist separator 92 and the cathode side mist separator 97 include a cylindrical vessel in which chemical-resistance mesh or porous material exists, or a unit in which the retention time of electrolytically generated gas is long enough for the mist to fall. Alternatively, effective means include that the saturated vapor pressure is lowered by cooling the gas-liquid separator system, the mist separator or piping connecting to these units; thereby moisture in the electrolytically generated gas is coagulated to enhance the moisture removal efficiency at the gas-liquid separator system or the mist separator so that the quantity of carrying over to the downstream lines is reduced.

If a large quantity of vapor or mist is included in the anode side gas-liquid separator 91, the cathode side gas-liquid separator 96, the anode side mist separator 92, and the cathode side mist separator 97, the gas passage will be blocked by the internally accumulated liquid and the gas may not be vented from the anode side tank 31, and the cathode side tank 38. To cope with this, it is desirable that the internally accumulated liquid should be periodically drained from the anode side gas-liquid separator 91, the cathode side gas-liquid separator 96, the anode side mist separator 92, and the cathode side mist separator 97.

Drain of the fluid from the anode side gas-liquid separator 91 and the anode side mist separator 92 is carried out via the anode side drain pipe 95 by opening the anode side gas pipe drain valve 94. Drain of the fluid from the cathode side gas-liquid separator 96 and the cathode side mist separator 97 is carried out via the cathode side drain pipe 100 by opening the cathode side gas pipe drain valve 99.

The following explains gas liquor separation; in particular, the functions of the respective equipment and the status at the operation of the apparatus for electrolyzing sulfuric acid.

(1) At the Dilution of Sulfuric Acid

The anode side cooler 34 is provided in the anode side dilute sulfuric acid generation loop A, but when sulfuric acid is diluted, the solution temperature rises higher than the room temperature. At this time, it is possible for the gas (air) at a water vapor pressure which is in equilibrium position in the concentration and temperature of the dilute sulfuric acid stored in the anode side tank 31 to exist in the gas space of the anode side tank 31, which is larger than that of the piping. In contact with the tank wall or piping wall, the gas containing water vapor is cooled down below the room temperature to form condensate of water drop.

For dilution of sulfuric acid, a specified quantity of pure water is provided first in the anode side tank 31 or the piping, to which a specified quantity of sulfuric acid is supplied and mixed during circulation. Thus, the liquid surface in the anode side tank 31 rises with the amount of supplied sulfuric acid, and the gas in the anode side tank 31 is gradually expelled outside the anode side tank 31, that is, the upper part of the anode side gas vent pipe 102, generating flow of air. Together with the flow of air, water droplet attached to the wall surface move in the anode side gas vent pipe 102.

(2) At the Electrolysis

During electrolysis, similar phenomenon to (1) occurs from heat generation by electrolysis, not from heat generation in (1) by dilution. In the configuration shown in FIG. 1, the heat generated from mixing pure water with sulfuric acid as in (1) is removed swiftly by the anode side cooler 34, whereas, it is presumed that the heat generated from electrolysis in the electrolytic cell 2 produces more water vapor (water drop) than the case (1) because it raises the temperature of the electrolyte which is supplied to the anode side tank 31. Moreover, during electrolysis, electrolytically generated gas occurring from the electrode by electrolysis is included in the electrolyte in fine bubble. The fine bubbles, which move from the electrolyte into the gas phase in the anode side tank 31 generate fine splash when they burst open on the liquid surface to form mist contained in the gas in the anode side tank 31.

(3) Each Mechanism

The anode side mist separator 92 connected to the upper part of the anode side gas-liquid separator 91 is able to separate the mist from the gas, which is droplet floating in the gas because of fineness, by means of the micro porous separation membrane preventing the mist from passing through. As the mist separated by the separation membrane increases, it becomes droplet, which can flow as liquid. The liquid a separated by the anode side gas-liquid separator 91 flows down the gas-liquid separator system 91 by gravity (self-weight). The mist separated by the anode side mist separator 92 collects to be droplet, flows down the anode side mist separator 92 by self-weight and moves to the anode side gas-liquid separator 91. The liquid b separated by the anode side mist separator 92 flows down the anode side gas-liquid separator 91 as with the liquid a. The liquid a and the liquid b having flown down the anode side gas-liquid separator 91 collect before the anode side gas pipe drain valve 94 and drain out of the apparatus by self-weight when the anode side gas pipe drain valve 94 is opened. Thus, for draining the liquid separated by the gas-liquid separator system or the mist separator, relations of height among units are important. It is necessary, at least, to do it sequentially with the anode side mist separator 92, the anode side gas-liquid separator 91, and the anode side gas pipe drain valve 94 from the upper part.

Anode side gas pipe drain valve 94 can be opened and closed at any arbitrary timing. The same is preferably applied to the cathode side mist separator 97 connected at the upper part of the cathode side gas-liquid separator 96.

In order to efficiently carry out the drain of liquid in the gas-liquid separator system and the mist separator, it is recommendable to apply the difference of pressure. For instance, liquid of sulfuric acid in the gas-liquid separator system and the mist separator can be efficiently drained by providing a pressure reducer (not shown in the figures) to the anode side drain pipe 95 and the cathode side drain pipe 100 to reduce the pressure in the gas-liquid separator system and the mist separator so that the gas flow in an opposite direction to the flow of the anode gas and the cathode gas is created.

As an alternative way, the inert gas supply part (not shown in the figures) is provided at the outlet side of the anode side mist separator 92 and the cathode side mist separator 97. By blowing inert gas into the gas-liquid separator system and the mist separator in an opposite direction to the flow of the anode gas and the cathode gas, sulfuric acid solution in the gas-liquid separator system and the mist separator can be drained efficiently. As inert gas, nitrogen gas, for example, is applicable.

As the diaphragm 5 used for the electrolytic cell 2, hydrophilically treated porous fluororesin membrane or fluororesin type cation exchange membrane is desirable. In case that the fluororesin type cation exchange membrane is applied, such phenomena are observed that the concentration of sulfuric acid at the anode will increase with lapse of electrolysis time by the effect of water entrained when cation permeates through the ion exchange membrane from the anode side to the cathode side; the quantity of liquid at the anode side will decrease; the concentration of sulfuric acid at the cathode will decrease by dilution with entrained water; and the quantity of liquid at the cathode will increase.

The quantitative control of liquid at the cathode side is performed by open close operation of the cathode tank drain valve 113. The liquid is drained by self-weight outside the apparatus by opening the cathode tank drain valve 113 in the both cases of periodical drain and regulating by the height of the liquid surface of the tank. A variety of method can be applied to administrate the quantity of drain. For instance, a liquid surface sensor to measure the low position is provided to the cathode side tank 38 and by using it, the cathode tank drain valve 113 is closed when the liquid surface of draining liquid has reached the sensor position. For the timing of opening the cathode tank drain valve 113, such options are available that the valve is opened when the entrained water volume calculated from the electrolysis time and electric current supply has reached the preset level or that the liquid surface of accumulated entrained water has reached the sensing position preset in a liquid surface sensor to measure the high position provided to the cathode side tank 38.

Whereas, the balancing gas for the space in the cathode side tank 38 can be introduced from the cathode gas scrubber 98 through the cathode side gas vent pipe 103.

If the catholyte increases due to the entrained water, the liquid surface of the cathode side tank 38 rises and exceeds the capacity of the cathode side tank 38. In order to prevent excessive storage, the cathode tank drain valve 113 opens when the liquid surface of the cathode side tank 38 has reached the preset height and drains the specified quantity of liquid through the cathode tank drain pipe 112. The liquid surface of the cathode side tank 38 is regulated by means of the liquid surface sensor, etc., which are not shown in the figures. Thus, in the situation that the volume of dilute sulfuric acid solution at the cathode side electrolysis part 23 has increased due to the water entrained by cation migrated through the fluororesin type cation exchange membrane, the overflow of the cathode side tank 38 can be prevented by discharging a specified quantity of liquid periodically or at the time when the liquid surface of the cathode side tank 38 has reached the predetermined height.

Whereas, if operation is continued without discharging the catholyte, the sulfuric acid of the cathode is further diluted with entrained water and lowered concentration leads to significantly poor conductivity. When operation is continued without replacing catholyte for a long time, the concentration can be controlled to a constant level in such a manner that the sulfuric acid concentration of catholyte is monitored by a sulfuric acid concentration meter, which is not shown in the figures and concentrated sulfuric acid is supplemented as required from the cathode side concentrated sulfuric acid supply part 39.

For instance, the concentration of sulfuric acid in catholyte can be regulated in such a way that the volume of entrained water is calculated from the measurements of electrolysis time and electric current; the concentration of sulfuric acid of electrolyte at the time when the volume of calculated entrained water is added to the volume of electrolyte in the cathode side tank 38 prepared before the electrolysis is calculated; when the calculated sulfuric acid concentration is lower than the specified range, the quantity of sulfuric acid to be added to restore in the specified range is calculated; and concentrated sulfuric acid is supplied to the cathode side electrolysis loop B′ in operation from the cathode side concentrated sulfuric acid supply 39 with quantitation of the calculated sulfuric acid quantity by a flow meter.

In order to minimize the fluctuation of electrolysis conditions, it is important that the injection speed of concentrated sulfuric acid is slowed so that the temperature and the concentration of sulfuric acid supplied to the cell are controlled to stay within the specified range.

In the anode side electrolytic sulfuric acid generation loop B of the anode side electrolysis part 20, the electrolytic sulfuric acid in the anode side tank 31 which has reached the specified concentration of oxidizing agent after the specified time of electrolysis is supplied to the location of use outside the apparatus via the anode tank drain pipe 110 and the anode tank drain valve 11. In the cathode side electrolysis loop B′ of the cathode side electrolysis part 23, electrolyte in the cathode side tank 38 after the specified time of electrolysis is drained outside the apparatus via the cathode tank drain pipe 112 and the cathode tank drain valve 113.

When the anode side tank 31 empties with no electrolytic sulfuric acid, adjustment of the concentration of sulfuric acid starts again. At this time, it is desirable for reducing the consumption of chemicals that the concentration of the catholyte is controlled and the characteristic values, such as the concentration or the conductivity of the catholyte are monitored so that the catholyte is repeatedly used as far as the characteristic values stay within the specified values. The administration of the temperature and the concentration at the cathode side are not directly related with the current efficiency of persulfuric acid, but still the administration of them is preferable for the following reasons. The catholyte transfers heat to the anolyte through the diaphragm 5, which is cation exchange membrane and impedes the anolyte temperature from staying within the specified range; in case that the concentration of anolyte is different from that of catholyte, the diaphragm 5, which is cation exchange membrane becomes the interface of the concentration difference of two electrolytes at which heat of dilution occurs, affecting the current efficiency of persulfuric acid due to difficulty in temperature control of the electrolyte; the diaphragm 5, which is cation exchange membrane degrades or dimensionally changes due to overheat; and bubbles of water vapor occur due to overheat, leading to a larger resistance of the cell. Thus, the administration of the temperature and the concentration at the cathode side are necessary.

FIG. 2 shows the processes for controlling the concentration of sulfuric acid and the electrolysis of the apparatus for electrolyzing sulfuric acid 1 as shown in FIG. 1. The processes in the anode side electrolysis part 20 comprise the processes as follows, as shown in FIG. 2.

1) Pure Water Supply Process

Pure water is supplied to the anode side tank 31 from the anode side pure water supply pipe 10.

2) Pure Water Circulation Process

Pure water is circulated by the anode side circulation pump 33. At this time, pure water is circulated in the anode side dilute sulfuric acid generation loop A via the anode side by-pass pipe 36, without being supplied to the anode compartment 4.

3) Concentrated Sulfuric Acid Supply Process

Concentrated sulfuric acid is supplied from the anode side concentrated sulfuric acid supply part 32 to the pure water circulating in the anode side dilute sulfuric acid generation loop A, in which the concentrated sulfuric acid and pure water is being mixed by continuous circulation. In this method, the solution enters the anode side cooler 34 right after the concentrated sulfuric acid is mixed with the pure water, and therefore, heat of dilution which occurs when concentrated sulfuric acid is mixed with pure water is immediately removed so that generation of vapor or mist is suppressed.

Moreover, temperature rise of the anode side concentrated sulfuric acid supply part 32 derived from heat of dilution is suppressed, which leads to the protection of piping, pumps, valves in the vicinity from damage or deformation due to high heat.

4) Gas Vent Piping Drain Process

Drainage of the anode side gas-liquid separator 91 and the anode side mist separator 92 is carried out by opening the anode side gas pipe drain valve 94 via the anode side drain pipe 95. This process is performed on an as-needed basis in the processes: 3) Concentrated sulfuric acid supply process, 5) Sulfuric acid temperature and concentration control process and 6) Electrolysis process.

5) Sulfuric Acid Temperature and Concentration Control Process

Dilute sulfuric acid solution is being circulated and mixed while being cooled down to the specified temperature or less, preferably to 30 degrees Celsius or less within the anode side dilute sulfuric acid generation loop A. The solution with the sulfuric acid temperature at 30 degrees Celsius or less gives a high current efficiency of oxidizing agent formation. Thus, it is preferable that the sulfuric acid temperature is cooled down to 30 degrees Celsius or less before electrolysis. Moreover, the sulfuric acid concentration is preferably controlled to 2˜10 mol/L. If it exceeds 10 mol/L, the current efficiency of oxidizing agent suddenly decreases to 60% or less; whereas, if it becomes below 2 mol/L, sulfuric acid ion, as raw material of oxidizing agent, in the solution decreases and the current efficiency degrades to 60% or less. Therefore, the sulfuric acid concentration should preferably be controlled to the range of 2˜10 mol/L.

While the processes progress in the anode side electrolysis part 20, the following processes progress also in the cathode side electrolysis part 23 as shown in FIG. 2.

1) Pure Water Supply Process

Pure water is supplied to the cathode side tank 38 from the cathode side pure water supply pipe 12.

2) Pure Water Circulation Process

Pure water is circulated in the cathode side dilute sulfuric acid generation loop A′ by the cathode side circulation pump 40. At this time, pure water is circulated in the cathode side tank 38 via the cathode side by-pass pipe 43, without being supplied to the cathode compartment 7.

3) Concentrated Sulfuric Acid Supply Process

Concentrated sulfuric acid is supplied from the cathode side concentrated sulfuric acid supply part 39 to the pure water circulating in the cathode side dilute sulfuric acid generation loop A′, in which the concentrated sulfuric acid and pure water are being mixed by continuous circulation. In this method, the solution enters the cathode side cooler 41 right after the concentrated sulfuric acid is mixed with the pure water, and therefore, heat of dilution which occurs when concentrated sulfuric acid is mixed with pure water is immediately removed so that generation of vapor or mist is suppressed. At this time, if the supply amount of concentrated sulfuric acid is 25% or less to the circulating amounts, temperature rise of the cathode side concentrated sulfuric acid supply part 39 derived from heat of dilution is suppressed, which leads to the protection of piping, pumps, and valves in the vicinity from damage or deformation due to high heat.

4) Gas Vent Piping Drain Process

Drainage of the cathode side gas-liquid separator 96 and the cathode side mist separator 97 is carried out by opening the cathode side gas pipe drain valve 99 via the cathode side drain pipe 100. This process is performed on an as-needed basis in the processes: 3) Concentrated sulfuric acid supply process, 5) Sulfuric acid temperature and concentration control process and 6) Electrolysis process.

5) Sulfuric Acid Temperature and Concentration Control Process

Dilute sulfuric acid solution is being circulated and mixed till it becomes evenly while being cooled down to a specified temperature or less, preferably to 30 degrees Celsius or less within the cathode side dilute sulfuric acid generation loop A′. The solution with the sulfuric acid temperature at 30 degrees Celsius or less gives a high current efficiency of oxidizing agent formation. Thus, it is preferable that the sulfuric acid temperature is cooled down to 30 degrees Celsius or less before electrolysis. Moreover, the sulfuric acid concentration is preferably controlled to 2˜10 mol/L. If it exceeds 10 mol/L, the current efficiency of oxidizing agent sharply decreases to 60% or less; whereas, if it becomes below 2 mol/L, sulfuric acid ion, as raw material for oxidizing agent, in the solution decreases and the current efficiency degrades to 60% or less. Therefore, the sulfuric acid concentration should preferably be controlled to the range of 2˜10 mol/L.

Because the anode side is completely separated from the cathode side, the processes 1)-5) carried out in the anode side and the cathode side are the same and are performed completely independently. Dilute sulfuric acid controlled to the specified temperature and the concentration in the anode side dilute sulfuric acid generation loop A of the anode side and in the cathode side dilute sulfuric acid generation loop A′ of the cathode side is electrolyzed in the electrolysis process of the anode side electrolytic sulfuric acid generation loop B and the cathode side electrolysis loop B′.

6) Electrolysis Process

Electrolysis process is the process to electrolyze dilute sulfuric acid solution which is carried out after the completion of the processes: 1˜5) at the both sides of the anode and the cathode. Electrolysis is performed by circulating dilute sulfuric acid solution in the both anode side electrolysis part 20 and cathode side electrolysis part 23. A high current efficiency is achieved when the temperature of the solution is at 30 degrees Celsius or less. Therefore the temperature of the solution during the electrolysis is preferably controlled to 30 degrees Celsius or less.

7) Anolyte (Electrolytic Sulfuric Acid) Supply Process

Electrolytic sulfuric acid formed in the electrolysis process is controlled to the specified temperature and the specified concentration in the anode side electrolytic sulfuric acid generation loop B of the anode side electrolysis part 20 and supplied to a location of use outside the system. This is called the electrolytic sulfuric acid solution supply process. More specifically, after electrolysis is performed in the electrolysis process for the specified time, the electrolytic sulfuric acid solution supply process transfers anolyte to the location of use outside the system when the concentration of oxidizing agent, which is being observed by a concentration monitor not shown in the figures, has reached to the specified level.

Electrolytic sulfuric acid is supplied to a resist stripping apparatus, an etching apparatus or the like, but the apparatus or equipment to be connected are not specifically restricted.

In the apparatus for electrolyzing sulfuric acid by the present invention, a concentration monitor to measure the concentration of oxidizing agent or sulfuric acid can be built in the apparatus or provided on the external piping through which electrolytic sulfuric acid flows. Measurements obtained from the concentration monitor are able to be used to control electric current supplied to the electrolytic cell or to determine the timing of the output of operation signal, solution transfer signal or alarms supplied to the equipment, such as cleaning units, to which electrolyte sulfuric acid is sent from the apparatus for electrolyzing sulfuric acid. The measuring method of the concentration monitor is not specifically restricted.

8) Catholyte Drain Process

When the catholyte increases during the electrolysis process due to entrained water and the liquid surface in the cathode side tank 38 reaches the specified position, the cathode tank drain valve 113 is temporarily opened to drain a small amount of catholyte.

The catholyte formed in the electrolysis process is drained from the cathode side electrolysis loop B′ of the cathode side electrolysis part 23, which is called the catholyte drain process. The catholyte drain process drains the full amount of catholyte diluted by entrained water from the cathode side tank 38. Drainage may be performed when the time of use of catholyte has reached the pre-determined time or may be performed when the concentration of sulfuric acid of the catholyte as measured by the sulfuric acid concentration meter, which is not shown in the figures, is known to have decreased to the specified value. Whereas, the cathode drain process can be performed at the same time as the anolyte supply process, but not as the electrolysis process.

In other examples of the present invention, two or more of anode side tanks can be installed in the anode side electrolysis part 20. As an example, an individual function can be assigned to each tank, such as the tank exclusively used for transferring the solution outside the apparatus, the tank exclusively used for controlling the dilute sulfuric acid, and the tank exclusively used for the electrolysis, or differently such as the tank exclusively used for transferring the solution outside the apparatus and the tank exclusively used for the control of the dilute sulfuric acid and the electrolysis process, so that sulfuric acid including oxidizing agent can be efficiently formed in a large amount for a short period of time. In the cathode side electrolysis part 23, also, it is similarly possible to provide such system having multiple tanks. In the apparatus for electrolyzing sulfuric acid 1, two or more of the electrolytic cell 2 are allowed to be installed. Also possible is that two or more of the anode-cathode pairs can be installed to one electrolyzer to form a bipolar configuration.

FIG. 3 shows an example of installing plural anode side tanks in the anode side electrolysis part 20. Though the cathode side electrolysis part 23 is not shown, the configuration is same as the cathode side electrolysis part 23 shown in FIG. 1. FIG. 3 shows the apparatus for electrolyzing sulfuric acid in which the first anode side tank 49 and the second anode side tank 50 are installed in parallel in the anode side dilute sulfuric acid generation loop A. After Electrolytic sulfuric acid containing formed oxidizing agent is stored once in the first anode side tank 49, electrolisys process is switched to the second anode side tank 50 by the change-over valves 51˜58, where electrolytic sulfuric acid containing oxidizing agent at the specified concentration is formed. In this way, (1) electrolyzed sulfuric acid is stored in the first anode side tank 49 and while electrolytic sulfuric acid is similarly being manufactured in the second anode side tank 50 with related valves switched, electrolytic sulfuric acid can be supplied to the location of use from the first anode side tank 49. By repeating these operations, electrolytic sulfuric acid can be kept supplied continuously, and at the same time, (2) it is possible to manufacture and store electrolytic sulfuric acid with different concentration of sulfuric acid•oxidizing agent separately in the first anode side tank 49 and the second anode side tank 50 and to supply to two different locations of use, or to supply to application processes which require a different oxidizing power from one apparatus. As afore-mentioned, multiple cathode side tanks are able to be installed in the cathode side electrolysis part 23 as with the case of the anode side tank.

FIG. 4 shows the processes of the sulfuric acid concentration control and the electrolysis in the apparatus for electrolyzing sulfuric acid 1 of FIG. 3, in which the case is given that only one cooler and one sulfuric acid mixer are provided for common use. First, the process shown at the left side of FIG. 4 is explained.

1) Pure Water Supply Process

Pure water is supplied to the anode side tank 49 via the anode side pure water supply pipe 10 by opening the change-over valve 55. Quantitation of supply water volume is available by closing the change-over valve 55 which operates on the signals from the liquid surface sensor provided to the first anode side tank 49 or from the integrating flow meter provided to the anode side pure water supply pipe 10. Whereas, the change-over valves 52 and 54 belonging to the anode side tank 50 are closed.

2) Pure Water Circulation Process

Pure water is circulated by operating the anode side circulation pump 33. At this time, the anode side by-pass valve 35 is opened, and the anode compartment outlet valve 22 and the anode compartment inlet valve 21 are closed. Pure water circulates in the anode side dilute sulfuric acid generation loop A via the anode side by-pass pipe 36, without being sent to the anode compartment 4.

3) Concentrated Sulfuric Acid Supply Process

Concentrated sulfuric acid is supplied from the anode side concentrated sulfuric acid supply part 32 to the pure water circulating in the anode side dilute sulfuric acid generation loop A, in which the concentrated sulfuric acid and pure water are being mixed by continuous circulation. In this method, the solution enters the anode side cooler 34 right after the concentrated sulfuric acid is mixed with the pure water, and therefore, heat of dilution which occurs when concentrated sulfuric acid is mixed with pure water is immediately removed so that generation of vapor or mist is suppressed. Moreover, temperature rise of the anode side concentrated sulfuric acid supply part 32 derived from heat of dilution is suppressed, which leads to the protection of piping, pumps, and valves in the vicinity from damage or deformation due to high heat.

4) Gas Vent Piping Drain Process

Drainage of the anode side gas-liquid separator 91 and the anode side mist separator 92 is carried out by opening the anode side gas pipe drain valve 94 via the anode side drain pipe 95. This process is performed on an as-needed basis in the processes: 3) Concentrated sulfuric acid supply process, 5) Sulfuric acid temperature and concentration control process and 6) Electrolysis process.

5) Sulfuric Acid Temperature and Concentration Control Process

Dilute sulfuric acid solution is being circulated and mixed while being cooled down to a specified temperature or less, preferably to 30 degrees Celsius or less within the anode side dilute sulfuric acid generation loop A. The solution with the sulfuric acid temperature at 30 degrees Celsius or less gives a high current efficiency of oxidizing agent formation. Thus, it is preferable that the sulfuric acid temperature is cooled down to 30 degrees Celsius or less before electrolysis.

Moreover, the sulfuric acid concentration is preferably controlled to 2˜10 mol/L. If it exceeds 10 mol/L, the current efficiency of oxidizing agent sharply decreases to 60% or less; whereas, if it becomes below 2 mol/L, sulfuric acid ion, as raw material for oxidizing agent, in the solution decreases and the current efficiency degrades to 60% or less. Therefore, the sulfuric acid concentration should preferably be controlled to the range of 2˜10 mol/L.

Dilute sulfuric acid controlled to the specified temperature and the concentration in the anode side dilute sulfuric acid generation loop A of the anode side is electrolyzed in the electrolysis process of the anode side electrolytic sulfuric acid generation loop B.

6) Electrolysis Process

Electrolysis process is the process to electrolyze dilute sulfuric acid solution carried out in succession of the processes of 1)˜5). Although FIG. 4 does not include the cathode side, the processes of 1)˜5) are performed also on the cathode side similarly to the anode side, which is same as in FIG. 2.

Electrolysis is performed in the anode side electrolysis part 20 by circulating dilute sulfuric acid solution. High current efficiency is achieved when the temperature of the solution is at 30 degrees Celsius or less. Therefore the temperature of the solution during the electrolysis is preferably controlled to 30 degrees Celsius or less. Dilute sulfuric acid solution is circulated between the anode side tank 49 and the anode compartment 4 by closing the anode side by-pass valve 35, and opening the anode compartment outlet valve 22 and the anode compartment inlet valve 21. Direct current is supplied to the electrolytic cell 2 and electrolysis is carried out at the specified supply current for the specified time duration to obtain electrolytic sulfuric acid containing oxidizing agent of the specified concentration. With the change-over valves 51 and 53 being closed, the formed electrolytic sulfuric acid containing oxidizing agent of the specified concentration is stored in the first anode side tank 49.

7) Anolyte (Electrolytic Sulfuric Acid) Supply Process

Electrolytic sulfuric acid formed in the electrolysis process is controlled to the specified temperature and the specified concentration in the anode side electrolytic sulfuric acid generation loop B of the anode side electrolysis part 20 and supplied to a location of use outside the system. This is called the electrolytic sulfuric acid solution supply process. More specifically, after electrolysis is performed in the electrolysis process for the specified time, the electrolytic sulfuric acid solution supply process supplies anolyte to the location of use outside the system when the concentration of oxidizing agent, which is being observed by a concentration monitor not shown in the figures, has reached to the specified level.

Electrolytic sulfuric acid is supplied to a resist stripping apparatus, an etching apparatus or the like, but the apparatus or equipment to be connected are not specifically restricted.

In parallel with 7) Anolyte supply process, pure water is supplied similarly to 1) to the anode side tank 50 as shown on the right side of FIG. 4, followed by the processes of 1)→6).

1) Pure Water Supply Process

Pure water is supplied to the anode side tank 50 via the anode side pure water supply pipe 10 by opening the change-over valve 56. Quantitation of supply water volume is available by closing the change-over valve 56 which operates on the signal from the liquid surface sensor provided to the second anode side tank 50 or the signal from the integrating flow meter provided to the anode side pure water supply pipe 10. Whereas, the change-over valves 52 and 54 belonging to the anode side tank 50 are opened.

2) Pure Water Circulation Process

Pure water is circulated by operating the anode side circulation pump 33. At this time, the anode side by-pass valve 35 is opened, and the anode compartment outlet valve 22 and the anode compartment inlet valve 21 are closed. Pure water circulates in the anode side dilute sulfuric acid generation loop A via the anode side by-pass pipe 36, without being sent to the anode compartment 4.

3) Concentrated Sulfuric Acid Supply Process

Concentrated sulfuric acid is supplied from the anode side concentrated sulfuric acid supply part 32 to the pure water circulating in the anode side dilute sulfuric acid generation loop A, in which the concentrated sulfuric acid and pure water are being mixed by continuous circulation. In this method, the solution enters the anode side cooler 34 right after the concentrated sulfuric acid is mixed with the pure water, and therefore, heat of dilution which occurs when concentrated sulfuric acid is mixed with pure water is immediately removed so that generation of vapor or mist is suppressed. Moreover, temperature rise of the anode side concentrated sulfuric acid supply part 32 derived from heat of dilution is suppressed, which leads to the protection of piping, pumps, and valves in the vicinity from damage or deformation due to high heat.

4) Gas Vent Piping Drain Process

Drainage of the anode side gas-liquid separator 91 and the anode side mist separator 92 is carried out by opening the anode side gas pipe drain valve 94 via the anode side drain pipe 95. This process is performed on an as-needed basis in the processes: 3) Concentrated sulfuric acid supply process, 5) Sulfuric acid temperature and concentration control process and 6) Electrolysis process.

5) Sulfuric Acid Temperature and Concentration Control Process

Dilute sulfuric acid solution is being circulated and mixed while being cooled down to the specified temperature or less, preferably to 30 degrees Celsius or less within the anode side dilute sulfuric acid generation loop A. The solution with the sulfuric acid temperature at 30 degrees Celsius or less gives a high current efficiency of oxidizing agent formation. Thus, it is preferable that the sulfuric acid temperature is cooled down to 30 degrees Celsius or less before electrolysis.

Moreover, the sulfuric acid concentration is preferably controlled to the range of 2˜10 mol/L. The current efficiency of oxidizing agent is higher than the sulfuric acid of 10 mol/L or more; whereas, if it becomes below 2 mol/L, sulfuric acid ion, as raw material of oxidizing agent, in the solution decreases and the current efficiency degrades.

Dilute sulfuric acid controlled to the specified temperature and the concentration in the anode side dilute sulfuric acid generation loop A of the anode side is electrolyzed in the electrolysis process of the anode side electrolytic sulfuric acid generation loop B.

6) Electrolysis Process

Electrolysis process is the process to electrolyze dilute sulfuric acid solution carried out in succession of the processes of 1)˜5). Although FIG. 4 does not include the cathode side, the processes of 1)˜5) are performed also on the cathode side similarly to the anode side, which is same as in FIG. 2.

Electrolysis is performed in the anode side electrolysis part 20 by circulating dilute sulfuric acid solution. High current efficiency is achieved when the temperature of the solution is at 30 degrees Celsius or less. Therefore the temperature of the solution during the electrolysis is preferably controlled to 30 degrees Celsius or less. Dilute sulfuric acid solution is circulated between the anode side tank 50 and the anode compartment 4 by closing the anode side by-pass valve 35, and opening the anode compartment outlet valve 22 and the anode compartment inlet valve 21. Direct current is supplied to the electrolytic cell 2 and electrolysis is carried out at the specified supply current for the specified time duration to obtain electrolytic sulfuric acid containing oxidizing agent of the specified concentration. With the change-over valves 52 and 54 being closed, the formed electrolytic sulfuric acid containing oxidizing agent of the specified concentration is stored in the first anode side tank 50.

Then, electrolytic sulfuric acid is supplied to a location of use from the second anode side tank 50; whereas, pure water supply process starts again at the first anode side tank 49 and repeated.

EXAMPLE

Following explains the present invention more in detail citing examples and comparative examples. However, the present invention shall not be limited to the examples.

Example 1

The experiment was conducted using the apparatus for electrolyzing sulfuric acid and the method for electrolyzing sulfuric acid shown in FIG. 1 and FIG. 2. For the anode 3 and the cathode 6 incorporated in the electrolytic cell 2, a conductive diamond electrode was applied, which was prepared in such a way that diamond provided with conductivity by boron doping was coated on a 200 mm φ silicon substrate. Current density was 100 A/dm². For both the anode side and the cathode side, the sulfuric acid temperature and concentration control process was as follows, in which concentrated sulfuric acid was diluted with pure water to prepare dilute sulfuric acid of the specified temperature and the specified concentration.

The procedures for the anode side were as follows.

1) Pure water was supplied from the anode side pure water supply pipe 10 to the anode side tank 31 and stored. Feed amount of the pure water was weighed by using an ultrasonic integrating flow meter, which is not shown in the figures.

2) Pure water was circulated in the anode side dilute sulfuric acid generation loop A by operating the anode side circulation pump 33.

3) To the pure water circulating in the anode side dilute sulfuric acid generation loop A, concentrated sulfuric acid was supplied from the anode side concentrated sulfuric acid supply part 32 to form dilute sulfuric acid. Feed amount of the concentrated sulfuric acid was weighed by using an ultrasonic integrating flow meter, which is not shown in the figures.

4) Heat of dilution generated by mixing concentrated sulfuric acid with pure water was cooled down for temperature control to 30 degrees Celsius or less at the anode side cooler 34 during circulation; whereas diluted sulfuric acid solution resulting from dilution of concentrated sulfuric acid with pure water was sufficiently stirred and mixed by circulation.

The procedures for the cathode side were as follows.

1) Pure water was supplied from the cathode side pure water supply pipe 12 to the cathode side tank 38 and stored. Feed amount of the pure water was weighed by using an ultrasonic integrating flow meter, which is not shown in the figures.

2) Pure water was circulated in the cathode side dilute sulfuric acid generation loop A′ by operating the cathode side circulation pump 40.

3) To the pure water circulating in the cathode side dilute sulfuric acid generation loop A′, concentrated sulfuric acid was supplied from the cathode side concentrated sulfuric acid supply part 39 to form dilute sulfuric acid. Feed amount of the concentrated sulfuric acid was weighed by using an ultrasonic integrating flow meter, which is not shown in the figures.

4) Heat of dilution generated by mixing concentrated sulfuric acid with pure water was cooled down for temperature control to 30 degrees Celsius or less at the cathode side cooler 41 during circulation; whereas diluted sulfuric acid solution resulting from dilution of concentrated sulfuric acid with pure water was sufficiently stirred and mixed by circulation.

After completion of the sulfuric acid concentration control and temperature control in the anode side and the cathode side, the anode compartment inlet valve 21 and the anode compartment outlet valve 22 were opened and the anode side by-pass valve 35 was closed in the anode side to configure the anode side electrolytic sulfuric acid generation loop B; the cathode compartment inlet valve 24 and the cathode compartment outlet valve 25 were opened and the cathode side by-pass valve 42 was closed to configure the cathode side electrolysis loop B′; and electrolysis is carried out with dilute sulfuric acid solution being circulated and direct current being supplied to the electrolytic cell to form electrolytic sulfuric acid containing oxidizing agent.

Then, by the afore-mentioned method, the sulfuric acid concentration before the electrolysis was controlled to the range of 1.8˜16.7 mol/L by the sulfuric acid concentration control process. The same method was applied to catholyte of the cathode side to control the concentration. After cooling dilute sulfuric acid solution, electrolysis was carried out. The applied conditions were as follows.

Current density  100 A/dm² Anolyte  1.8 mol/L (EL-UM 98 mass %  2.0 mol/L sulfuric acid by Kanto  2.3 mol/L Chemical Co., Inc. was  3.7 mol/L diluted.)  5.3 mol/L  7.1 mol/L  9.2 mol/L 10.0 mol/L 11.5 mol/L 14.1 mol/L 15.4 mol/L 16.7 mol/L Catholyte Same as anolyte Anolyte quantity  8.5 L catholyte quantity   6 L Circulation flow rate in   3 L/min the anode during electrolysis Circulation flow rate in   4 L/min the cathode during electrolysis Electrolyte temp. 30 degrees Celsius or less Diaphragm For sulfuric acid conc. in electrolyte: 10.0 mol/L or more, hydrophilically treated porous PTFE membrane applied. For sulfuric acid conc. in electrolyte: 9.2 mol/L or less, cation exchange membrane applied.

Dilute sulfuric acid electrolyzed by the afore-mentioned procedures under the afore-mentioned conditions was sampled from the sampling pipe branching at the electrolysis part not shown in the figures and the total quantity of the oxidizing agent formed in the dilute sulfuric acid was measured by the KI titrimetry.

Table 1 shows measurements of the concentration of total oxidizing agent at the same volume capacity density at the temperature of the dilute sulfuric acid applied to the electrolysis. The concentration of sulfuric acid was 3.7 mol/L. When the temperature excelled 30 degrees Celsius, the concentration decreased.

TABLE 1 Concentration Volume capacity of total density Solution temperature oxidizing agent Test No. (Ah/L) (degrees Celsius) (mol/L) RUN 1 46 25 0.66 RUN 2 46 29 0.65 RUN 3 46 34 0.54 RUN 4 102 23 1.25 RUN 5 102 27 1.11 RUN 6 102 45 0.79

It has been found that the current efficiency obtained from the concentration of the total oxidizing agent significantly decreases from around 30 degrees Celsius and that in order to form oxidizing agent efficiently, electrolysis of dilute sulfuric acid controlled to 30 degrees Celsius or less is effective.

Table 2 gives the concentration of the total oxidizing agent and the current efficiency obtained from the sulfuric acid concentration of 1.8˜16.7 mol/L, where applied current density was 100 A/dm² and the volume capacity density was 25 Ah/L. The current efficiency obtained from the concentration of total oxidizing agent was 60% or more in the range of 2.0˜10.0 mol/L of the sulfuric acid concentration. It is found that in the range outside 2.0˜10.0 mol/L, the current efficiency sharply drops.

TABLE 2 Concentration Volume of Sulfuric acid Current capacity total oxidizing Current concentration density density agent efficiency (mol/L) (A/dm²) (Ah/L) (mol/L) (%) RUN 7 1.8 100 25 0.25 54 RUN 8 2.0 100 25 0.28 60 RUN 9 2.3 100 25 0.32 69 RUN 10 3.7 100 25 0.36 77 RUN 11 5.3 100 25 0.35 75 RUN 12 7.1 100 25 0.34 74 RUN 13 9.2 100 25 0.29 63 RUN 14 10.0 100 25 0.28 60 RUN 15 11.5 100 25 0.19 42 RUN 16 14.1 100 25 0.13 29 RUN 17 15.4 100 25 0.11 24 RUN 18 16.7 100 25 0.06 14

Table 3 shows the case in which the electrolyte was being cooled to keep 30 degrees Celsius even during electrolysis and the case in which cooling was stopped during the electrolysis and the electrolyte heated up to 51 degrees Celsius by the heat of electrolysis.

TABLE 3 Concentration of Sulfuric acid Volume Solution total concentra- Current capacity temperature oxidizing tion density density (degrees agent (mol/L) (A/dm²) (Ah/L) Celsius) (mol/L) RUN 19 3.7 100 190 30 1.51 RUN 20 3.7 100 190 51 0.72

As known from Table 3, the concentration of oxidizing agent was 1.51 mol/L in the case that cooling at 30 degrees Celsius was continued; whereas in the case that cooling was suspended during electrolysis and the temperature of electrolyte rose up to 51 degrees Celsius by heat generated by electrolysis, the concentration of oxidizing agent was only 0.72 mol/L, not achieving efficient electrolysis.

Comparative Example 1

Comparative Example 1 shows the case that the mixing point of concentrated sulfuric acid and pure water was inside the anode side tank, and neither the gas-liquid separator system nor the mist separator were provided. In the process of dilute sulfuric acid formation of Comparative Example 1, cooling was not performed as required, and troubles of the apparatus occurred.

In Comparative Example 1, 2.6 L of ultrapure water was supplied from the top of the tank and 5.9 L of 98 mass % sulfuric acid was supplied from the bottom of the tank in order to prepare 6 mol/L of dilute sulfuric acid solution. Both solutions were at room temperature. The flow rate for the supply of ultrapure water was 3 L/min and that of 98 mass % sulfuric acid was 0.2˜1 L/min.

Inside the tank, a large volume of vapor occurred due to the temperature rise of the solution caused by the heat of dilution which was generated through mixing ultrapure water with sulfuric acid. Due to this vapor, mist attached to the inner wall of the gas vent piping.

Upon completion of supplying 98 mass % sulfuric acid, the pump was operated and the solution was circulated between the tank and the heat exchanger to cool down the dilute sulfuric acid solution to 25 degrees Celsius. After cooling, electrolysis was started by circulating the solution between the tank and the electrolytic cell. The temperature of the solution during electrolysis was 27 degrees Celsius and the gas pressure in the gas vent piping of the anode and the cathode was 3−5 kPa. In ten minutes after the start of electrolysis, the gas pressure in the cathode tank sharply rose to an abnormal level of 200 kPa and circulation of the solution between the tank and the electrolytic cell was ceased.

At this time, liquid as condensate of mist and vapor occurring at the dilution of sulfuric acid stagnated in the filter provided between the cathode tank and the hydrogen combustion tower installed as the cathode side scrubber; the gas filter was blocked by the condensate; and the cathode gas stagnated between the cathode tank and the filter, leading to build-up of the high pressure.

After releasing the remaining pressure, the cell was disassembled, during which through-holes were found in cation exchange membranes.

INDUSTRIAL APPLICABILITY

According to the apparatus for electrolyzing sulfuric acid and the method for electrolyzing sulfuric acid by the present invention, sulfuric acid containing oxidizing agent can be formed at a high efficiency and safely by the electrolysis of dilute sulfuric acid prepared to the specified temperature and concentration in the apparatus, under the controlled temperature condition. Furthermore, the present invention provides the apparatus for electrolyzing sulfuric acid and the method for electrolyzing sulfuric acid which can produce highly concentrated oxidizing agent solution at a high current efficiency, which was not able to achieve by the conventional technologies, and can form oxidizing agent stably.

REFERENCE SIGNS LIST

-   -   A: Anode side dilute sulfuric acid generation loop     -   B: Anode side electrolytic sulfuric acid generation loop     -   A′: Cathode side dilute sulfuric acid generation loop     -   B′: Cathode side electrolysis loop     -   1: Apparatus for electrolyzing sulfuric acid     -   2: Electrolytic cell     -   3: Anode     -   4: Anode compartment     -   5: Diaphragm     -   6: Cathode     -   7: Cathode compartment     -   10: Anode side pure water supply pipe     -   11: Anode side pure water supply valve     -   12: Cathode side pure water supply pipe     -   13: Cathode side pure water supply valve     -   20: Anode side electrolysis part     -   21: Anode compartment inlet valve     -   22: Anode compartment outlet valve     -   23: Cathode side electrolysis part     -   24: Cathode compartment inlet valve     -   25: Cathode compartment outlet valve     -   27: Anode side concentrated sulfuric acid supply pipe     -   28: Anode side concentrated sulfuric acid supply valve     -   29: Cathode side concentrated sulfuric acid supply pipe     -   30: Cathode side concentrated sulfuric acid supply valve     -   31: Anode side tank     -   32: Anode side concentrated sulfuric acid supply part     -   33: Anode side circulation pump     -   34: Anode side cooler     -   35: Anode side by-pass valve     -   36: Anode side by-pass pipe     -   37: Anode side circulation pipe     -   38: Cathode side tank     -   39: Cathode side concentrated sulfuric acid supply part     -   40: Cathode side circulation pump     -   41: Cathode side cooler     -   42: Cathode side by-pass valve     -   43: Cathode side by-pass pipe     -   44: Cathode side circulation pipe     -   49: The first anode side tank     -   50: The second anode side tank     -   51˜58: Change-over valve     -   91: Anode side gas-liquid separator     -   92: Anode side mist separator     -   93: Ozone decomposition mechanism     -   94: Anode side gas pipe drain valve     -   95: Anode side drain pipe     -   96: Cathode side gas-liquid separator     -   97: Cathode side mist separator     -   98: Cathode gas scrubber     -   99: Cathode side gas pipe drain valve     -   100: Cathode side drain pipe     -   102: Anode side gas vent pipe     -   103: Cathode side gas vent pipe     -   110: Anode tank drain pipe     -   111: Anode tank drain valve     -   112: Cathode tank drain pipe     -   113: Cathode tank drain valve 

1. An apparatus for electrolyzing sulfuric acid wherein: in the apparatus for electrolyzing sulfuric acid 1 comprising the anode side electrolysis part 20 and the cathode side electrolysis part 23, the anode side dilute sulfuric acid generation loop A in which concentrated sulfuric acid, as feed material, is diluted and controlled to a specified temperature and concentration, and the anode side electrolytic sulfuric acid generation loop B in which the dilute sulfuric acid generated in the anode side dilute sulfuric acid generation loop A is electrolyzed to form electrolytic sulfuric acid and the formed electrolytic sulfuric acid is controlled to a specified temperature and concentration are provided at least in the anode side electrolysis part 20; in the anode side dilute sulfuric acid generation loop A, the anode side tank 31, the anode side concentrated sulfuric acid supply part 32 and the anode side cooler 34 are disposed in this order constituting a loop being connected by the anode side by-pass pipe 36, to which the anode side pure water supply pipe 10 is connected to feed pure water to the anode side dilute sulfuric acid generation loop A at any point wherever in the anode side dilute sulfuric acid generation loop A; the anode side concentrated sulfuric acid supply pipe 27 is connected to feed concentrated sulfuric acid to the anode side concentrated sulfuric acid supply part 32; in the anode side electrolytic sulfuric acid generation loop B, the anode side tank 31 and the anode compartment 4 provided internally with the anode 3 of the electrolytic cell 2 comprising the anode compartment 4 and the cathode compartment 7 separated by the diaphragm 5 constitute a loop being connected by the anode side circulation pipe 37; the concentrated sulfuric acid supplied to the anode side concentrated sulfuric acid supply part 32 from the anode side concentrated sulfuric acid supply pipe 27 is diluted with pure water supplied by the anode side pure water supply pipe 10; the diluted sulfuric acid of a low concentration is controlled to a specified temperature and concentration while being circulated in the anode side dilute sulfuric acid generation loop A to form dilute sulfuric acid of the specified temperature and concentration; the formed dilute sulfuric acid is supplied to the anode compartment 4 of the electrolytic cell 2 via the anode side circulation pipe 37 constituting the anode side electrolytic sulfuric acid generation loop B; electrolytic sulfuric acid is formed in the anode compartment 4 and the formed electrolytic sulfuric acid is controlled to a specified temperature and concentration while being circulated in the anode side electrolytic sulfuric acid generation loop B; and electrolytic sulfuric acid of the specified temperature and concentration is generated.
 2. The apparatus for electrolyzing sulfuric acid as defined in claim 1, wherein: in the cathode side electrolysis part 23, the cathode side dilute sulfuric acid generation loop A′ in which concentrated sulfuric acid, as feed material, is diluted to sulfuric acid of a low concentration and the sulfuric acid of a low concentration is controlled to a specified temperature and concentration, and the cathode side electrolysis loop B′ in which the dilute sulfuric acid formed in the cathode side dilute sulfuric acid generation loop A′ is circulated through the cathode compartment 7 are provided; in the cathode side dilute sulfuric acid generation loop A′, the cathode side tank 38, the cathode side concentrated sulfuric acid supply part 39 and the cathode side cooler 41 are disposed in this order, constituting a loop being connected by the cathode side by-pass pipe 43, to which the cathode side pure water supply pipe 12 is connected to feed pure water to the cathode side dilute sulfuric acid generation loop A′ at any point wherever in the cathode side dilute sulfuric acid generation loop A′ and the cathode side concentrated sulfuric acid supply pipe 29 is connected to feed concentrated sulfuric acid to the cathode side concentrated sulfuric acid supply part 39; in the cathode side electrolysis loop B′, the cathode side tank 38 and the cathode compartment 7 provided internally with the cathode 6 of the electrolytic cell 2 comprising the anode compartment 4 and the cathode compartment 7 separated by the diaphragm 5 constitute a loop being connected by the cathode side circulation pipe 44; the concentrated sulfuric acid supplied to the cathode side concentrated sulfuric acid supply part 39 from the cathode side concentrated sulfuric acid supply pipe 29 is diluted with pure water supplied via the cathode side pure water supply pipe 12; the diluted sulfuric acid of a low concentration is controlled to a specified temperature and concentration while being circulated in the cathode side dilute sulfuric acid generation loop A′ to form dilute sulfuric acid of the specified temperature and concentration; the formed dilute sulfuric acid is supplied to the cathode compartment 4 of the electrolytic cell 2 via the cathode side circulation pipe 44 constituting the cathode side electrolysis loop B′; and the dilute sulfuric acid controlled to the specified temperature and concentration is electrolyzed while being circulated in the cathode side electrolysis loop B′.
 3. The apparatus for electrolyzing sulfuric acid as defined in claim 1, featuring that above the anode side tank 31, the anode side gas-liquid separator 91 and the anode side mist separator 92 are connected sequentially in series via the anode side gas vent pipe 102, and the respective bottoms of the anode side gas-liquid separator 91 and the anode side mist separator 92 are provided with a draining means of a communicating vessel structure for the common use by the anode side gas-liquid separator 91 and the anode side mist separator 92 to drain liquid accumulated at the bottom of the respective separators.
 4. The apparatus for electrolyzing sulfuric acid as defined in claim 2, featuring that above the anode side tank 31, the anode side gas-liquid separator 91 and the anode side mist separator 92 are connected sequentially in series via the anode side gas vent pipe 102, and the respective bottoms of the anode side gas-liquid separator 91 and the anode side mist separator 92 are provided with a draining means of a communicating vessel structure for the common use by the anode side gas-liquid separator 91 and the anode side mist separator 92 to drain liquid accumulated at the bottom of the respective separators, and above the cathode side tank 38, the cathode side gas-liquid separator 96 and the cathode side mist separator 97 are connected sequentially in series via the cathode side gas vent pipe 103, and the respective bottoms of the cathode side gas-liquid separator 96 and the cathode side mist separator 97 are provided with a draining means of a communicating vessel structure for the common use by the cathode side gas-liquid separator 96 and the cathode side mist separator 97 to drain liquid accumulated at the bottom of the respective separators.
 5. The apparatus for electrolyzing sulfuric acid as defined in claim 3 featuring that the ozone decomposition mechanism 93 is connected to the anode side mist separator
 92. 6. The apparatus for electrolyzing sulfuric acid as defined in claim 4 featuring that the hydrogen treatment mechanism is connected to the cathode side mist separator
 97. 7. The apparatus for electrolyzing sulfuric acid as defined in claim 1 featuring to have such configuration that more than one of anode side tank are installed in parallel in the anode side dilute sulfuric acid generation loop A, generated electrolytic sulfuric acid containing oxidizing agent is once stored in one of the tanks, then the valve is switched to other anode side tanks where electrolytic sulfuric acid containing oxidizing agent of the specified concentration is formed.
 8. The apparatus for electrolyzing sulfuric acid as defined in claim 7 featuring to have such configuration that while electrolytic sulfuric acid containing oxidizing agent of the specified concentration stored in one anode side tank is being transferred to a location of use outside the apparatus for electrolyzing sulfuric acid, electrolytic sulfuric acid containing oxidizing agent of the specified concentration is formed in another anode side tank.
 9. The apparatus for electrolyzing sulfuric acid as defined in claim 1 featuring that the anode 3 is a conductive diamond electrode.
 10. The apparatus for electrolyzing sulfuric acid as defined in claim 1 featuring that the diaphragm 5 is a fluororesin type cation exchange membrane or a hydrophilically treated porous fluororesin membrane.
 11. A method for electrolyzing sulfuric acid wherein electrolytic sulfuric acid controlled to the specified temperature and concentration is formed by applying the apparatus as defined in claim
 1. 12. The method for electrolyzing sulfuric acid wherein the apparatus as defined in claim 1 is applied, a porous fluororesin membrane is applied as the diaphragm 5, and overflowing of the cathode side tank 38 is prevented by draining periodically or draining a specified quantity when the liquid level of the cathode side tank 38 has reached a specified point because of an increase of the dilute sulfuric acid solution circulating in the cathode side electrolysis loop B′ of the cathode side electrolysis part 23 due to entrained water brought by cation at the time of passing through the porous fluororesin membrane.
 13. The method for electrolyzing sulfuric acid wherein the apparatus as defined claim 1 is applied, a porous fluororesin membrane is applied as the diaphragm 5, and concentrated sulfuric acid is replenished to the cathode side concentrated sulfuric acid supply part 39 in order to maintain a certain range of dilute sulfuric acid concentration when the sulfuric acid concentration of the dilute sulfuric acid solution formed in the cathode side dilute sulfuric acid generation loop A′ of the cathode side electrolysis part 23 has decreased to or below the specified level of concentration due to entrained water brought by cation at the time of passing through the porous fluororesin membrane.
 14. The method for electrolyzing sulfuric acid as defined in claim 11 featuring that the temperature of dilute sulfuric acid before electrolysis in the anode side dilute sulfuric acid generation loop A of the anode side electrolysis part 20 or in the cathode side dilute sulfuric acid generation loop A′ of the cathode side electrolysis part 23 is controlled to 30 degrees Celsius or less.
 15. The method for electrolyzing sulfuric acid as defined in claim 11 featuring that the temperature of electrolyzed electrolyte in the anode side electrolytic sulfuric acid generation loop B of the anode side electrolysis part 20 or in the cathode side electrolysis loop B′ of the cathode side electrolysis part 23 is controlled to 30 degrees Celsius or less.
 16. The method for electrolyzing sulfuric acid as defined in claim 11 featuring that the sulfuric acid concentration of dilute sulfuric acid before electrolysis in the anode side dilute sulfuric acid generation loop A of the anode side electrolysis part 20 or in the cathode side dilute sulfuric acid generation loop A′ of the cathode side electrolysis part 23 is controlled to the range of 2˜10 mol/L. 